1. Field of the Invention
The present invention relates to an improved process for carrying out liquid phase oligomerizations and etherifications in a fixed catalyst bed.
2. Related Art
Recently a new method of conducting certain catalytic reactions has been devised. Two particular reactions for which this method has been particularly useful are oligomerization and etherification of C.sub.4 and C.sub.5 isoolefins. The method involved is briefly described as one where concurrent reaction and distillation occur in a combination reactor-distillation column with the distillation structure serving as the catalyst. This process and catalytic distillation structures are described in several U.S. Patents, namely U.S. Pat. Nos. 4,242,530; 4,250,052; 4,232,177; 4,302,356; 4,307,254; and 4,336,407.
This new system has been commercially applied to the production of methyl tertiary butyl ether (MTBE) produced by the reaction of isobutene contained in C.sub.4 refinery streams and methanol.
It is well known that primary alcohols will react preferentially with the tertiary alkenes in the presence of an acid catalyst, for example, as taught in U.S. Pat. Nos. 3,121,124; 3,629,478; 3,634,534; 3,825,603; 3,846,088; 4,071,567; and 4,198,530.
The catalytic distillation process differs from these older processes in that a catalyst system was disclosed (U.S. Pat. Nos. 4,215,011 and 4,302,356) which provided and distillation concurrently in the same reactor, at least in part within the catalyst system. For example, in this system and procedure, methanol and an isobutene containing C.sub.4 stream are continuously fed to the reactor/distillation column where they are contacted in the catalytic distillation structure. The methanol preferentially reacts with isobutene, forming MTBE, which is heavier than the C.sub.4 components of the feed and the methanol, hence it drops in the column to form the bottoms. Concurrently, the unreacted C.sub.4 's (e.g. n-butane, n-butenes) are lighter and form an overhead.
The reaction just described is reversible, which means that it is normally equilibrium limited, however, by removing the ether (MTBE) from contact with the catalyst (as a bottoms), the reaction is forced to completion (Le Chatelier Principle). Hence it can be run to obtain a very high conversion of the isobutene present (95%+). As a result, a great deal of control over the rate of reaction and distribution of products can be achieved by regulating the system pressure. The boiling point in the reactor is determined by the toiling point of the lowest boiling material (which could be an azeotrope) therein at any given pressure. Thus at a constant pressure a change in the temperature at a point within the column indicates a change in the composition of the material at that point. Thus to change the temperature in the column the pressure is changed for any given composition.
Since oligomerization and etherification are exothermic there is excess heat in the reactor. In the liquid phase system, methods were devised to remove this heat, since in the case of resin type catalysts excessive temperature (hot spots) can damage the catalyst. In the catalytic distillation the excess heat merely causes more boil up in the column.
In an unrelated area the volatilization of a portion of the feed in a catalytic C.sub.3 hydrogenation to provide a quasi-isothermal reactor is discussed in Chemical Engineering Progress, Vol 70, No. 1, January, 1974, pages 74-80.
In addition to the catalytic distillation system, there are several other etherification systems in commercial use or available which are liquid phase systems. That is, these systems are operated under conditions of pressure to maintain the contents of the reactor in liquid phase. One other principal problems encountered in these systems is the exothermic heat of reaction. Heat is sometimes removed by using heat exchangers in the reactor, such as tubular reactors having a heat exchange medium contacting the tubes, other systems employ feed diluents to maintain a low concentration of reactive isobutene. In other words, the temperature in the catalyst bed has to be controlled by the removal of excess heat in some manner.
The present invention which relates to the liquid phase type of reaction also provides a means for removing heat from the fixed continuous catalyst bed. It is a further advantage that the present type of liquid phase reaction may be used in conjunction with a catalytic distillation column reactor to obtain very high conversions of iso C.sub.4 and C.sub.5 alkenes in the feed stream.
These and other advantages will become apparent from the following descriptions.